Coating substrates with reinforced resin matrices, such as liquid resins reinforced with fibers, glass microspheres, or other reinforcing or filler materials (hereinafter referred to as reinforcing material), conventionally requires mixing the liquid resin with the reinforcing material and then painting or spraying the mixture onto the substrate, or dipping the substrate into the mixture. When only a portion of the substrate requires coating, accuracy and control requirements typically dictate the use of a spray coating process. Spray coating processes, however, are limited due to the low sprayability of high performance liquid resins which are typically highly viscous, the limit in attainable coating thickness, and the high amount of waste material generated.
Many liquid resins utilized in spray coating processes possess viscosities of about 20,000 centipoise (cps) or greater. At such high viscosities, pumping the liquid resin through the lines and nozzle of a spray coating apparatus is difficult and requires large amounts of energy. In order to reduce energy requirements and to simplify the spray coating process, the viscosity of the liquid resin is often reduced to about 2,000 cps by mixing the liquid resin with a solvent. Typically, however, solvents useful in spray coating processes are generally environmentally hazardous. Consequently, waste material from the spray coating process must be disposed of as hazardous waste.
Conventional spray coating processes comprise combining a liquid resin, flow leveling and spray solvents, reinforcing material, and other conventional constituents such as curing agents, biocides, catalysts, etc., in a tank to form a mixture. This mixture is then pumped from the tank through lines to a nozzle where it is atomized and sprayed onto the substrate. Once the mixture has been applied to the substrate, the flow leveling solvents are removed therefrom by the natural evolution of volatile gas and/or by applying heat to the mixture to hasten the solvent evolution.
During the flow leveling solvent evolution, solvent near the substrate surface migrates to the coating surface, dragging liquid resin with it, and thereby forming resin starved areas in the coating. These resin starved areas result in poor adhesion between the coating and the substrate, and act as potential coating failure points. The effect of the solvent migration can be minimized by applying thinner coatings, less than about 0.04 inches (0.10 cm), to the substrate. However, thick coatings of about 0.25 inches (0.64 cm) to about 0.50 inch (1.27 cm) or greater, are often required to attain the desired substrate protection, such as thermal protection.
An additional disadvantage of these coating processes relates to pot life. Since all of the coating constituents are combined in a tank and pumped through the coating system as a single mixture, there is limited time available to process and apply the coating. During the pumping, the liquid resin can begin to set up within the system and the reinforcement can accumulate within the lines or the nozzle, both resulting in a clogged nozzle and/or lines. Additionally, any unused portion of the batch must be disposed of as hazardous waste due to the presence of the hazardous solvents.
U.S. Pat. No. 5,307,992, to Hall et al. discloses an improved coating system and process where the liquid resin and reinforcing material are mixed external to the nozzle, thereby virtually eliminating clogging problems and significantly reducing system waste. The end effector used therein, however requires a separate gas line and utilizes an air disc to carry the reinforcing material to the liquid resin. These components render the end-effector large, difficult to maneuver, and impractical to use in confined spaces.
What is needed in the art is an improved end-effector for a convergent spray coating apparatus and process.